1. Technical Field
The invention relates generally to detector devices, methods for making the same, and methods for using the same. In particular, the invention relates to a thermopile infrared detector, a thermopile infrared detector array, and a method of manufacturing and using the same. Specifically, the invention relates to a thermopile infrared detector containing improved heat sinks for thermocouples in the detectors.
2. Background of the Invention
Thermopile infrared detectors are being implemented in consumer and automotive applications due to their low cost and relatively simple process technologies. A thermopile is a set of n thermocouples connected in parallel thermally and in series electrically.
A thermocouple converts the thermal difference over its two junctions into a voltage difference. By heating a junction between two conductors, a temperature difference between the junction and a portion located away from the junction is produced, thereby generating a diffusion current. A reverse electromotive force (called the Seebeck voltage) is produced to compensate for the diffusion current. By measuring the Seebeck voltage, the temperature difference between the two ends of the thermocouple can be obtained.
The value of the Seebeck voltage is determined from the product of the temperature difference between the two ends of the thermocouple and the Seebeck coefficient of the two conductors forming the thermocouple. The thermally generated voltage can be amplified by connecting plural pairs of thermocouples in series to form a thermopile. Therefore, the Seebeck voltage of the thermopile is equal to a value that is obtained from the product of the Seebeck voltage of a single thermocouple and the number of the thermocouples in series.
FIG. 1 is a schematic illustration showing a conventional thermopile infrared detector array. Only one complete thermopile detector 10 is shown in FIG. 1. As depicted in FIG. 1, the thermopile detector 10 includes a substrate 100 and a suspending membrane 101 that is formed on the substrate 100 with a plurality of thermocouples 102. A hot junction 103 is located in a middle portion of the suspending membrane 101, while a cold junction 104 is located in a peripheral portion of the suspending membrane 101 attached to the substrate 100 that acts as a heat reservoir. A plurality of etching windows 105 are formed on the suspending membrane 101. A polysilicon sacrificial layer (not shown) under the suspending membrane 101 can be etched via the etching windows 105 to construct a suspending structure. The structure of the thermopile detector 10 will be better understood with reference to FIG. 2, which is a cross-section view taken along a line 2—2 of FIG. 1.
As depicted in FIG. 2, the thermopile detector 10 includes a substrate 100 and a suspending membrane 101. The substrate 100 includes an integrated circuit 107. Typically, a cavity 106 is formed between the suspending membrane 101 and the substrate 100. The suspending membrane 101 generally includes a first dielectric layer 108, a P-type polysilicon 102a, a second dielectric layer 109, an N-type polysilicon 102b, a third dielectric layer 110, and a metallic wiring 102c. The metallic wiring 102c connects the P-type polysilicon 102a with the N-type polysilicon 102b. A thermocouple 102 is composed of the P-type polysilicon 102a, the N-type polysilicon 102b, and the metallic (i.e., aluminum) wiring 102c. The regions of both the hot junction 103 and the cold junction 104 are also shown in this figure.
Thus, the thermopile structure uses several polysilicon-aluminum thermocouples connected in series to create a thermopile array. However, the silicon substrate has a high thermal conductivity, making it difficult to maintain a large temperature gradient. Therefore, the membrane, with a high thermal resistance, is used to thermally isolate the sensing element from the bulk of the silicon wafer resulting in very low heat capacity. The thermoelectric materials are supported on the membrane over the cavity in the silicon substrate, maximizing the thermal isolation of the thermocouple junctions from the substrate.
Typical thermopile configurations are arranged in squares with equal numbers of thermocouples on each side. Each thermocouple typically contains a pair of traces: an aluminum trace and a polysilicon trace. However, this results in outer aluminum traces with a polysilicon trace adjacent only one side, e.g., a polysilicon trace is not on both sides of the aluminum trace. The adjacent polysilicon traces have a significantly larger mass than the aluminum traces, thereby aiding in the transfer of excess heat from the aluminum traces during transient thermal conditions.
The thermopile is highly sensitive to Electrically Fast Transients (EFT)/Electrostatic Discharge (ESD). This sensitivity occurs because the aluminum traces that make up one leg of each thermocouple are very narrow in width. This results in less mass, thus lowering the amount of heat energy required to melt the trace. Therefore, the aluminum trace has low power dissipation capabilities, limiting the amount of energy that may be imparted to the thermopile without causing failure.
Since the membrane has a high thermal resistance, the aluminum traces cannot conduct the excess heat energy to the membrane. This also limits the ability of the outer aluminum traces to conduct transient heat. Therefore, the aluminum lines will fuse due to Joule heating at lower ESD voltages. The failure mechanism allows excessive buildup of temperature and heat, leading to electromigration of thermally-induced stress of the aluminum traces.
One way to overcome this problem is to make the aluminum traces wider. Larger aluminum widths, however, will have an adverse effect on the thermal conductance and the electrical resistance. This adversely effects the operation of the thermopile.